The bearings are mechanical components that guide the rotation of a shaft in a bearing assembly while limiting the friction effects that could be brought about by the movement of one of the two parts in relation to the other. The bearings are formed by two coaxial rings, one being the inner ring and the other the outer, between which mobile elements are placed and held. These mobile elements, which are generally balls, although trapped between the two coaxial rings, allow the rotation of one of the rings in relation to the other.
In some models, the balls are replaced by cylindrical or tapered rollers. The bearings are then capable of supporting a higher radial force in relation to the conventional ball bearings. Likewise, some bearings, described as needle bearings, employ rollers of small diameter compared to their length, giving them the advantage of being less bulky by virtue of a reduced radial space.
And yet, although the use of bearings reduces the friction effects due to the rotation of a shaft in its bearing assembly, fatigue in the mechanical components will appear once a certain number of rotations has been exceeded. This deterioration reaches the rolling parts such as the rings. The deterioration can take the form of normal wear, surface chipping, corrosion, seizing, abrasion, etc., which then generates an impact, or can take the form of an unbalance in the shaft causing a dynamic lack of balance. This deterioration in the condition of the mechanism then leads to vibration, which increases with the wear.
It is therefore known that if the increase in the vibrations is used to detect a fault, then examining the characteristics of the vibration spectrum of the machine can be used to identify its cause and therefore to determine the time remaining until the critical threshold will be reached. This vibration varies according to the type of damage to the mechanism. Unbalance conditions from an unbalance in the shaft produces a sinusoidal excitation while surface chipping on a track in a bearing brings about a shock wave that leads to pulse-type excitation at the passage of each of the mobile elements of the bearing over the irregularity.
Currently, the main method employed to characterise and monitor the state of each essential component in a rotary machine consists of using vibration sensors of the accelerometric type. The phenomenon exploited in this type of sensor is called piezoelectricity. Under the action of a mechanical force, certain bodies can become polarised. In order to exploit this property, the piezoelectric sensors take the form of a disk, in which each of the surfaces is connected to an electrode. A pressure on one face of the sensor generates a mechanical stress that polarises the sensor. The charge generated is then amplified so that it can be measured.
In order that the piezoelectric sensors can measure the vibrations due to damage, the piezoelectric sensors are positioned close to strategic points of the main components of the monitored machine. The frequency of the vibrations in the bearing leads to stress/pressure frequencies at the surface of the sensor, which are then converted into the form of a variation in an electrical test signal.
It emerges that the use of such sensors has many drawbacks. In addition to their high cost, these sensors cannot always be positioned as close as necessary to the source at the origin of any vibration. However, the vibrations engendered by the faults in the bearing have the particular feature of propagating throughout the structure of the machine. These vibrations can thus change medium due to a change in the nature of the materials, which then gives rise not only to refraction or reflection phenomena, but also to conversion of the propagation mode. It is therefore important, in order to correctly measure the vibration in a machine, that the sensors are positioned at optimal measurement points. In the case where use is made of acceleration-measuring or piezoelectric sensors, reaching these optimal measurement points is not always possible. Unfortunately, the vibration is damped or attenuated as it moves away from the source at which it was generated. The placement of these sensors at a distance from the source of the vibration then results in a significant attenuation of the measured signal.
In addition, it is necessary to note that these vibration sensors have a measurement quality that depends on the surface against which they are positioned. This surface must be able to correctly transmit the measured vibration so that there is no loss of information